This invention relates to electronic control systems for electric motors. More particularly, this invention relates to a pulse-width modulated (PWM) control system.
Direct current (DC) brushless motors are attracting growing attention for a wide variety of industrial applications. The DC brushless motors are often used in servo control systems. A DC brushless motor generally includes a permanent magnet rotor or a wound rotor and a set of stator windings. The windings are commutated by solid state switches controlled in response to a signal from a rotor position sensor, which is directly related to the back EMF (electro-motive force) of the motor.
At low speeds, it is preferable to commutate the motor such that the current supplied to the stator windings is "in phase" (zero phase difference) with the back EMF of the rotor. However, as operating speed is increased in the motor, it then becomes desirable to provide a phase advance or a leading current to the stator windings. The leading current compensates for the inductance of the stator windings as well as the back EMF generated by the windings.
In the simplest form of providing phase advance, commutation is adjusted as a function of rotor speed to provide the leading current. For example, many controllers now incorporate a digital signal processor or other microcontroller to control commutation of the switching devices. A lookup table can be provided to store phase advance values. The microcontroller accesses the lookup table as a function of rotor speed and obtains the desired phase adjustment.
In yet more sophisticated embodiments, the lookup table can be two dimensional. As appreciated by those skilled in the art, the preferred phase adjustment for a leading current may be a function of both rotor speed and the load on the motor. Typically, motor load is related to the amplitude of the current provided to the stator windings. The lookup table thus can provide values indicative of phase adjustment as a function of both rotor speed and motor load.
However, a significant disadvantage of lookup tables is that the values must be predetermined in order to provide the proper phase advance. In other words, motor parameters, such as the inductance of the motor and the back EMF coefficient and/or other related parameters such as drive bus voltage must be assumed, premeasured or otherwise ascertained to calculate the correct phase adjustment to be entered in the lookup table. Of course, if the calculated or assumed values are in error, motor performance suffers. Likewise, if the operating characteristics of the motor change, the lookup table values are again in error.
In view of the shortcomings of current designs, there is a continuing need for an improved motor control system that can selectively generate a leading current.